I ruled it the way a number of other people have... if you screw the suppressor onto the gun, in order to function it covers the vents of the gas vent. This is true of any weapon that comes with a built-in gas vent, should you choose to suppress it. Weapons with built-in suppressors, should you choose to vent them, essentially have the vent built into the top of the suppressor in such a way that you can open and close it, providing (again) either suppression or venting, but not both.

From Raygun's Glossary (because Dictionary.Com isn't good at firearms terminology).

Gas vent: "A device on the muzzle of a firearms which diverts some of the emerging gas and forces it upwards, so developing a downward thrust to counteract the rize of the weapon muzzle during firing."

Sound suppressor: "Device attached to the barrel of a weapon and which traps the emergent gas and forces it to pass round a series of baffles, cooling it and thus reducing its volume, so that it emerges at low speed and does not, therefore, make much noise."

The gas vent requires high velocity propellant gases that it can vent away from the firearm to counter the (effects of) recoil. A sound suppressor traps the propellant gases and slows them down.

Check out Raygun's Suppression page, the Wiki articles on sound suppressors and muzzle brakes, etc. etc.

Are somantic components something used by rules lawyer mages to argue their point?